Помогите дорешать уравнение под б)

Решите задачу:

$\frac{sin(- \alpha )+cos(\pi + \alpha )}{1+2cos(\frac{\pi}{2}- \alpha )cos(- \alpha )} = \frac{-sin \alpha -cos \alpha }{1+2sin \alpha \cdot cos \alpha } = \frac{-(sin \alpha +cos \alpha )}{sin^2 \alpha +cos^2 \alpha +2sin \alpha \cdot cos \alpha } =\\\\= \frac{-(sin \alpha +cos \alpha )}{(sin \alpha +cos \alpha )^2} =-\frac{1}{sin \alpha +cos \alpha }=A\; ;$

$sin \alpha +cos \alpha =sin \alpha +sin(\frac{\pi}{2}- \alpha )=\\\\=2sin\frac{ \alpha +\frac{\pi}{2}- \alpha }{2}\cdot cos\frac{ \alpha -\frac{\pi}{2}+ \alpha }{2}=2sin\frac{\pi}{4}\cdot cos( \alpha -\frac{\pi}{4})=\\\\=2\cdot \frac{\sqrt2}{2}\cdot cos( \alpha -\frac{\pi}{4})=\sqrt2\cdot cos( \alpha -\frac{\pi}{4})\\\\\\A=-\frac{1}{2\sqrt2cos( \alpha -\frac{\pi}{4})}$